1. Field of the Invention
The present invention relates to a measuring apparatus for determining the density of fluids, comprising a buoyant member immersed in the fluid to be investigated and having a known specific weight, whose buoyancy is balanced out via a loading device having a progressively-acting counterforce, and a measuring instrument for the vertical position of the buoyant member which comprises a capacitor which is variable in its value of capacitance corresponding to the vertical position of the buoyant member and an arrangement for monitoring the value of capacitance that serves as a measure for the fluid density.
2. Description of the Prior Art
Measuring apparatus for determining the density of fluids that work upon utilization of the bouyancy of a measuring member are known in the art, for example, in the form of an aerometer or, respectively, of the Mohr-Westphal balance, while utilizing the Archimedean principle, allow relatively simple and accurate density calculations under defined, standardized preconditions. In the known embodiments of such apparatus, problems arise, first of all, by reading or, respectively, further interpretation of the measurement results and, secondly, by measurements of fluids whose density changes in time-dependent fashion, so that, for example, the measurement of flowing fluids whose densities change over time is not possible or is only possible with disproportionately great expense.
In order to be able to determine density changes essentially continuously and of a fluid at least flowing with a certain flow rate as well, the Soviet Union patent document SU-A-765 705, for example, discloses a measuring device of the type initially set forth that works in a buoyancy-dependent manner and that is fashioned such that the fluid under investigation flows through a specimen container from the bottom to the top, whereby the buoyant member immersed into the fluid is balanced out via a suspension wire with a load device arranged outside of the specimen container. Whereas the suspension wire is guided with a constant radius relative to the pivot point of the load device independently of the buoyancy-dependent position of the buoyant member, the suspension of a counterweight at the load device is executed such that the effective lever arm increases linearly with increasing immersion depth of the buoyant member, so that the vertical position thereof in the fluid provides a direct measure for the respective density of the fluid. In addition to being identified by reading a scale, the respective density can also be identified with this known apparatus via a variable disk capacitor adjusted with the load device or, respectively, via the capacitance thereof.
The German patent document DE-A1-36 32 019, for example, discloses a device for measuring the density, whereby a measuring spindle is immersed into a measuring chamber, and a proximity switch that outputs an analog signal proportional to the density of the fluid is attached under the measuring spindle.
Although a continuous monitoring of the density of even flowing fluid is possible with such apparatus, disadvantages result. For example, the mechanical execution, having a buoyant member suspended via a long, freely-swinging suspension and having a load weight or a floating measurement spindle requires great care in the implementation of the measurement or in the stabilization of the overall measuring instrument. This is necessary since only the stationary utilization of the stabilized measuring instrument is meaningful, particularly for more accurate calculations of density. The same disadvantages, moreover, are also present in view of the initially-mentioned, previously-known modifications of density measuring equipment that work upon the utilization of the Archimedean principle. This results in certain employments of density measurements at fluids that are currently critical cannot be undertaken with the aforementioned, known apparatus.
In recent years, therefore, the continuous monitoring or, respectively, calculation of the density of the fuel supplied, for example, to an internal-combustion engine has become particularly critical since this can be additionally employed as a parameter in electronically-controlled ignition systems. Therefore, for example, specifically lighter fuel is employed in northern European countries than in southern European countries. There is also usually a difference in density between what are referred to as summer fuels and winter fuels. When, for example, what is referred to as the blocking quantity (corresponds to "wide-open throttle") is designed such that the respective internal-combustion engine produces its full power given light-weight fuel, it will then smoke given heavy fuel and vice-versa. The maximum injection amount could be adapted to the fuel present in the fuel tank via density monitoring. For the above reasons, however, the mentioned, known arrangements are not suitable for introduction into and for operation in, for example, a truck.
An electronic density measuring device was, in fact, disclosed some time ago in which the fluid to be investigated is sucked into a bent U-shaped glass tube that, excited to mechanical oscillations in the KHz range, serves a resonator. The resonant frequency is then a measure for the density of the fluid located in the glass tube. Although such a measuring device can, of course, also be fundamentally introduced into a motor vehicle or the like and could be used for monitoring the density of the fuel serving for drive, there are critical disadvantages. Not only are the mechanical structure of the resonator tube (tolerances) and the oscillatory excitation and evaluation involved, but also a separate pump would also usually be necessary for sucking the fuel under investigation through the measuring instrument. This would cause high overall added expense, particularly for mass employment in, for example, production-line passenger automobiles and which, therefore, is economically unjustifiable.